The somatomedins are a group of peptides homologous to insulin which appear to have a major role in growth hormone action. Growth hormone has a major role in post natal somatic growth, but is not crucial for fetal growth. However, somatomedin is not dependent on growth hormone in fetal life. This project will test the hypothesis that the somatomedin peptides play a crucial role in fetal growth. Somatomedin will be measured at baseline and after experimental manipulation in fetal rabbits, fetal sheep, and fetal monkeys. The experiments will include hypophysectomy, nephrectomy, thyrodectomy and infusion of hormones and other pharmacologic agents. The somatomedin peptides produced by fetuses, fetal tissues, and tumor cell lines will be characterized by RRA, RIA column chromatography and electrophoresis. The development of receptors for somatomedins and biological response to somatomedins will be studied in fetal tissues in tissue culture. The role of other hormones in the control of receptors and biological response will also be studied in vitro. The experimental fetal and in vitro results will be correlated with findings in normal human newborns, premature newborns and newborn infants with growth disturbances. Those studies should provide insight into the hormonal control of fetal growth.